One of the more subtle features of the desiccation ridge process is that when ridges abut furrows they usually do so on the inside of the arch. Why this should be so, I don’t know. As I say, I’m fairly sure that the furrows represent a momentary “stall” in the flow of cement across the surface of the substrate. But why ridges tend to abut these furrows, and why usually on the inside?

As with a number of these features, it took me a while to notice this subtle feature. But when I became aware of it, I would notice it quite often. Take a look at one of the very first test casts I made. You can see that the ridges abut the major furrow on the inside of the arch:

Another early test cast:

Here is one made with clay-like soil from the Duwamish river:

A latter, more foot-shaped test cast. This is near the “heel” of the cast:

Here is an early test, using calcined kaolin as a test substrate:

Here is one using silica as a test substrate:

Another patch of desiccation ridges from the same cast:

The “Trigfoot” test cast, made in pumice:

By now, you should be able to see how common this feature is. In fact I’ve been able to find only two instances in all my test casts that did not follow this pattern. Here is one made in volcanic ash. The band of ridges is on the outside of the arch in this test cast. But notice too, that the furrow is fairly subtle in the first place:

Here is another test cast in which ridges form on both sides of the furrow:

The point of all this is that it is VERY common for ridges to abut furrows, and almost always the ridges form on the inside of the arch. It is this characteristic that I believe is the most telling characteristic that the CA-19 cast exhibits desiccation ridges and not dermal ridges: